1. Field of the Invention
The present invention relates to a drop-and-continue device that can easily add capabilities best suited for video, voice, and entertainment distribution, and other added value features, onto a Dmux/Mux type wavelength-division multiplexing (WDM) device.
2. Description of the Related Art
FIG. 1 is a diagram for explaining an overview of a cable television/video distribution network constructed to implement video, voice, and entertainment services provided by such entities as a cable television company or an MSO (Multiple Service Operator) that operates many cable television stations. In the figure, reference numeral 1 is an antenna of a television broadcasting station, 12 is a head end station, 13 to 15 are distribution nodes, 16 is an IP network, and 17 is a tail end station. The head end station 12 comprises a CATV distribution server 121, which receives radiowaves being radiated from the antenna 1 and transmits television signals to the distribution nodes 13 to 15 served by the head end, and a video distribution server 122, which transmits video signals to the IP network 16. The head end station 12 is, for example, an MSO (Multiple Service Operator) or a cable television company. The tail end station 17 is located at the terminal end of this distribution network.
Each distribution node, for example, the distribution node 13, comprises a transmission device 131, which transfers the incoming signal on to the next distribution node 14 or the tail end station and also to the end users 133 served by the distribution node 13, and a signal converter, for example, a quadrature amplitude modulator (QAM) 132, which converts the signal or data output from the transmission device 131 into a signal or data that can be used at the end user. The tail end station 17 also comprises a transmission device 171 and a quadrature amplitude modulator (QAM) 172.
The media (for example, television programs) to be provided to the end users are distributed from the CATV distribution server 121 in the head end station 12 to the end users via the respective distribution nodes. The media such as television programs distributed here are the same for all end users. Therefore, providing service of equal quality efficiently and simultaneously to all end users by a single distribution is of great importance to the MSO (Multiple Service Operator) or the cable television company.
FIG. 2 is a diagram for explaining the drop-and-continue function that each distribution node should have in order to provide the above service. As shown in the figure, it is strongly desired that the distribution node 13 be equipped with the drop-and-continue function that drops the received data A at the end user side and transfers the same data A to the next node.
Further, in such applications, data distribution from the end user side to the head end station or to the distribution node is quite unlikely (is not needed at all in the case of ordinary services but is only needed in the case of on-demand services recently put in operation), and it can therefore be said that the network is a one-way network. Accordingly, it is often practiced to save capital expenditures by using expensive backbone equipment only for service providing purposes and the IP network 16, etc. for data distribution from the end user side.
FIG. 3 is a block diagram showing the configuration of a cable television/video distribution network that uses metro WDM, that has become a mainstream technology recently, in order to implement the drop-and-continue function shown in FIG. 2. In the figure, reference numeral 31 is a head end station comprising a WDM 311 and a server 312, and 32 is a WDM which transfers a WDM multiplexed signal received from the WDM 311 on to a quadrature amplitude modulator (QAM) 322 via a Gigabit Ethernet (registered trademark) (GbE). The distribution node 33 is identical in configuration to the distribution node 32. Data output from the distribution node 33 returns to the WDM 311 in the head end station via a number of distribution nodes.
Utilizing the transparency of the WDM that can multiplex any data, regardless of its content, at the optical level for transmission, the prior art system shown in FIG. 3 enables the backbone network to be shared among a plurality of applications such as voice (telephone) and data services, and thus aims to efficiently increase the variety of services that an MSO or a cable television company can offer.
This, however, has made it difficult to implement the drop-and-continue function which is crucial to the implementation of the originally intended distribution service. The drop-and-continue function can be implemented using an expensive WDM called a new generation WDM that incorporates a reconfigurable switch, but replacing the existing Mux/Dmux type WDM equipment with the expensive WDM is not realistic from the standpoint of cost. Accordingly, it is strongly desired to provide the distribution service efficiently while making use of the existing Mux/Dmux type WDM.
FIG. 4 is a block diagram of a system, showing one implementation example of the drop-and-continue function that uses a Mux/Dmux type WDM according to the prior art. In the figure, reference numeral 41 is a wavelength division device (also called an optical demultiplexer or a DMUX) which separates multiplexed optical signals on a channel-by-channel basis according to the wavelength, 42 is a splitter which splits the optical signal of each channel output from the DMUX 41 into two parts, one to the end user side and the other to the WDM at the next stage, 43 is an amplifier which amplifies the output of the splitter 43, 44 is a multiplexing device (also called an optical multiplexer or a MUX) which multiplexes the output optical signal of the amplifier 43 with optical signals output from other amplifiers (not shown), 45 is a transponder which relays the optical signal separated by the splitter 42 and directed to the end user side, and 46 is a QAM which applies quadrature amplitude modulation to the output of the transponder 45 and passes the modulated signal to the end user side.
According to the prior art system shown in FIG. 4, with the optical splitter 42 inserted between the DMUX 41 and the MUX 44, an optical signal of wavelength λX can be dropped and the optical signal of the same wavelength λX can be transmitted to the next node.
FIG. 5 is a block diagram, of a system, showing another implementation example of the prior art drop-and-continue function that uses a Mux/Dmux type WDM according to the prior art. In the figure, reference numeral 51 is a wavelength division device (also called an optical demultiplexer or a DMUX) which separates multiplexed optical signals on a channel-by-channel basis according to the wavelength, 52 is a transponder which relays an optical signal output from the DMUX 51 to the end user side, 53 is a QAM which applies quadrature amplitude modulation to the output of the transponder 45 and passes the modulated signal to the end user side, and 54 is a wavelength multiplexing device (also called an optical multiplexer or a MUX) which receives, via the transponder 52, the optical signal looped back within the QAM or on the input side of the QAM and multiplexes the optical signal with optical signals from other transponders.
In FIG. 5, in the case of example (1), the signal output from the DMUX 51 and relayed via the transponder 52 is passed to the end user side, while the same signal is looped back within the QAM 53 and transferred via the transponder 52 to the MUX 54 for multiplexing.
In FIG. 5, in the case of example (2), the data (GbE) output from the transponder 52 is looped back before entering the QAM 53, and is transferred via the transponder 52 to the MUX 54 for multiplexing.
FIG. 6 is a block diagram showing, in further detail, the prior art system of FIG. 5 in the case of example (2). In the figure, the same component elements as those in FIG. 5 are designated by the same reference numerals. Reference numeral 61 is a network controller, 62 is a WDM device in the head end station, and 63 is the existing WDM device in the distribution node. The transponder 52 comprises an optical receiver 521 which receives an optical signal, for example, of 10 gigabits from the DMUX 51 and converts it into an electrical signal, a Gigabit Ethernet (registered trademark) splitter 522 which splits the electrical signal into a maximum of eight ports of 1-gigabit signals, a GbE transmitter 523 which converts the maximum of eight pieces of split data into optical signals for transmission to the QAM 53, a GbE receiver 524 which receives the optical signals looped back from the QAM 53 and converts them into electrical signals, a GbE multiplexer 525 which multiplexes the maximum of eight pieces of data supplied from the GbE receiver 524, and an optical receiver 526 which receives the multiplexed data and converts it into an optical signal. As shown, a fiber loopback is performed within the QAM 53.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2004-056685
[Patent Document 2] Japanese Unexamined Patent Publication No. H04-167634
[Patent Document 3] Japanese Unexamined Patent Publication No. H09-036834
In the prior art system shown in FIG. 4, which requires that the splitter 52 be inserted between the DMUX 41 and the MUX 44, the output level of the optical signal drops through the splitter 52; accordingly, the amplifier 43 must be inserted between the splitter 42 and the MUX 44 in order to adjust the output level. In the case of WDM, as the performance is determined by the worst wavelength of the multiplexed wavelengths, the amplifier must be inserted, for that wavelength, in order to compensate for the level drop. The insertion of the amplifier causes significant problems such as the degradation of OSNR (Optical Signal Noise Ratio) and the greatly increased equipment cost because of the need to provide the amplifier for each wavelength (i.e., for each channel). Therefore, in reality the system shown in FIG. 4 is hardly practicable.
The case (1) of FIG. 5 has the problem that the equipment configuration of the QAM is expensive, because the signal is looped back within the QAM 53. Furthermore, the transponder 52 must be fully equipped with circuits in both directions. This not only increases the failure rate, but also leads to a higher incidence of faults. A further problem is that it takes much labor to implement this function because an optical fiber or a cable has to be installed manually between the transponder 52 and the QAM 53 for each channel.
In the case (2) of FIG. 2, the output data (for example, GbE) of the transponder 52 is looped back, but this can only achieve the effect of omitting the loopback function in the QAM 53. In this case, a protocol for terminating the data (for example, GbE) and transmitting the data anew has to be handled within the transponder 52 and, hence, has the disadvantage that a new function has to be added to the transponder 52. Furthermore, since the data is looped back via a cable, there arises the problem that the work and management becomes complex.
Though not shown here, if the drop-and-continue function is not used at all, the same data will have to be broadcast via as many routes as there are distribution nodes, resulting in inefficient utilization of resources (wavelength, data, etc.). On the other hand, if the same resources (wavelength, data, etc.) are to be shared among them, one possible method would be to transmit the data at staggered intervals of time, but this would involve such problems as the work at each node increases because of signal route changing, etc. (or high-performance equipment such as DADM must be provided) and that there occurs a time difference (difference in quality) between the end users in delivering the service.
FIG. 7 is a diagram for explaining a further problem associated with the prior art systems shown in FIGS. 4 to 6. In the figure, reference numerals 71 to 74 are terminals connected to the distribution nodes served by the head end station. For example, as shown in part (2) of the figure, it is assumed that channel Ch1 is used between the terminals 71 and 72, channel Ch2 between the terminals 71 and 73, and channel Ch3 between the terminals 73 and 74. In this situation, if data on channel Ch3, for example, is to be handled by the drop-and-continue function, as the channel Ch3 is used at the terminal 73, the channel Ch3 in use must be released, for example, by changing the channel from channel Ch3 to channel Ch2 in order to implement the drop-and-continue function. As a result, channel reallocation must be done at the terminals 73 and 74. As this can cause an interruption in service or an error can occur in the work of channel reallocation, there arises the problem that not only does the reliability drop but the maintenance cost increases due to the channel reallocation.
The drop-and-continue function can also be implemented by using a new channel Ch4 as shown in part (1) of the figure, but this has the problem that efficiency in wavelength utilization drops and equipment cost increases because of the increase in the number of channels.